Trucks, buses and other such vehicles typically use air brake systems. These brake system includes air actuated service brakes coupled to service brake actuators. Pressurized air, typically at 100 psi, is applied to the service brake chambers of the service brake actuators to apply the service brakes. To keep the brakes applied while parked, combination brake actuators are usually used. The combination brake actuators include a spring brake portion and a service brake portion. The spring and service brake portions include respective spring and service brake chambers. The spring brake portion also includes a heavy actuator spring coupled to the brake through an actuator piston. The actuator spring tends to apply the brakes. Supplying pressurized air to the service brake chambers applies the brakes (as discussed above) while supplying pressurized air to the spring brake chambers moves the actuator piston to compress the actuator spring to release the brakes. Thus, when parked, air is exhausted from both the spring brake chambers and the service brake chambers which allows the actuator springs to apply the brakes according to the force of the actuator springs.
One of the problems with these conventional air brake systems is that the braking force generated by the spring brake portion of the combination brake actuator is only about 50% of the maximum braking force generated by an applied service brake. Therefore, each axle with combination brake actuators has only about half of the braking force which is available with the service brake. Also, not all of the axles have combination brake actuators; often no more than half of the axles are so equipped. Although the resulting braking force is sufficient for parking purposes, in an emergency when service brake air pressure is lost, the braking force available is woefully inadequate.